The separation of components from mixtures is a common problem in many industrial situations. A number of methods have been developed for separating the components of various mixtures from one another and typical methods include filtration, centrifugation, extraction and sedimentation.
Filtration generally comprises passing a discontinuous mixture through a barrier which is porous to one component but impenetrable to a second component, generally because the sizes of the individual particles or agglomeration of particles in the two components is different enough to make separation possible. One disadvantage of filtration, however, is that where the components are discontinuous but the individual particles or agglomeration of particles are approximately the same size, any barrier which will pass one component may also pass the other and any barrier which will stop one component may also stop the other. Accordingly, filtration is not appropriate for all types of separations.
Centrifugal separation is based on the difference between either the absolute mass or density of the different components of a discontinuous mixture. Consequently, the forces which are applied to a mixture in a centrifugal separator affect particles of different masses or densities to a greater or lesser extent so that they can be separated based on the degree to which they are affected. Typically, the forces exerted in a centrifugal separator will carry different components to different portions of the separator. Nevertheless, where the difference in absolute mass or density is not significant enough to discriminate between particles or agglomerations of particles, centrifugal separation will be ineffective. Additionally, because centrifugal separators operate on high speed rotation, they are an energy-intensive method of separation.
In extraction techniques, a discontinuous mixture is further mixed with a solvent in which one of the components of the mixture is miscible and in which the other component is immiscible. When mixed with the extraction solvent, the miscible component enters the extraction solvent and when the extraction solvent is then separated from the mixture, it carries the miscible component with it, thus providing the separation. Extraction technology depends, however, on the availability of appropriate solvents, is not always practicable for all types of separation requirements, and requires energy input to remove the miscible component from the solvent.
Sedimentation, as the name implies, simply allows a heavier component to settle out from a mixture under the influence of gravity. While very effective in certain circumstances, sedimentation is essentially a batch process, can require large amounts of time and extensive storage facilities and requires energy to either recover the liquid or remove the sediment, or both.
Yet other types of separation are the coalescence techniques, which are based partially on sedimentation and partly on the tendency of like materials to agglomerate or "grow" when given the opportunity to come into contact with one another. In typical coalescence techniques, a discontinuous mixture of a fluid continuous phase and either a fluid or solid discontinuous phase are encouraged to flow in a laminar manner while being provided with opportunities to collide with each other or with wettable coalescent barriers; e.g. barriers amenable to being wetted by those droplets. As used herein, the term laminar flow represents a continuous flow of fluid characterized by the lack of turbulence. By avoiding turbulence, a gravitationally-influenced separation can take place even while the mixture is flowing.
In coalescence techniques, individual droplets of a fluid are initially attracted towards one another by Van der Waals forces and agglomerate to form small clusters of droplets. This process is known as "flocculation". At some stage the attraction between the droplets becomes greater than the surface tension so that the droplets lose their individual "skins" and merge or "coalesce" into an even larger droplet.
As is known to those familiar with fluid behavior, droplets have a "skin" of molecules bound together by surface tension, a configuration dictated by the thermodynamic tendency of materials to reach their state of lowest potential energy. The magnitudes of surface tension forces grow progressively smaller as individual coalesced droplets grow larger and exhibit correspondingly larger surface areas. Thus, the larger and larger droplets more easily continue to coalesce into even larger droplets, and finally into pools of separated liquids.
Where turbulence is avoided, flocculation and coalescence--and hence separation--all take place more efficiently.
The rate at which or the ease with which coalescence takes place also depends to a large extent upon the population and size distribution of the particles or droplets of the discontinuous (sometimes referred to as the "dispersed") phase. Particles or droplets on the order of 1 mm or more in diameter generally settle or rise rapidly on their own. At the other extreme, particles with diameters on the order of 5-10 microns or less settle or rise very slowly and are often referred to as "permanent" or "stable" emulsions, because they are least affected by gravitational forces. Particles 4-5 microns or less in diameter enter the Brownian motion regime caused by the molecular vibration of the continuous phase. Below 0.5 microns, the particles behave almost as if in solution and normally cannot be separated by gravitational forces unless they collide, flocculate and/or coalesce.
In some typical coalescing techniques and equipment, a discontinuous mixture in which the continuous phase is a liquid is passed through a container having a number of continuous stacked parallel plates therein. The parallel plates keep the flow laminar as the mixture flows from the inlet to the outlet portions of the equipment. The plates generally have some sort of openings or "weep holes" in them so that given the laminar nature of the flow and the influence of gravity, the less dense component may move upwardly. Similarly, if the plates are properly tilted, any more dense discontinuous component, solid or liquid, will settle and slide downward.
In a further refinement of such devices, the plates are formed into specific shapes, generally sinusoidal configurations, i.e. S-shaped curves, so that individual particles or droplets in the components of a mixture flowing sinusoidally in a laminar fashion through such a device will eventually intercept one of the surfaces of one of the plates. Sinusoidal plates also provide more surface area than do more planar surfaces. Ideally, when an individual particle or droplet of the discontinuous phase approaches one of the curving surfaces one of several possible effects will take place: first, it may either immediately meet like particles and agglomerate or floc with them, or the droplet may travel along the surface until it meets a like particle and can agglomerate. Where the discontinuous phase is a liquid, it will eventually form a pool at the apex of each of the curves. The agglomeration accomplishes the separation as each agglomeration activity comprises the growth of a more distinct phase out of what was originally a mixture. As droplets move upwardly through the weep holes or downwardly along the inclined surface they continue to agglomerate and concentrate so that an overall separation takes place after a certain amount of travel, called "retention time" or "residence time", between such plates has taken place.
As a second possibility, the droplets of a liquid discontinuous phase may collide with other droplets and form clusters which, because of their relative bulk density, rise faster. The collision often ruptures the interfacial tension "skin" which surrounds and forms droplets of all types, making it easier for the droplet material to agglomerate or grow into larger, more bouyant droplets.
Finally, the particles may "wet" the surface which interrupted their flow, thereby remaining on the surface and providing a layer or film of separated component with which further particles will come in contact, agglomerate and hence separate. In short, the approaching particles "see" the film of like material rather than the surface underneath. This layer in turn will flow in a particular direction, depending on its relative density with respect to the continuous phase.
Although such techniques and equipment for accomplishing phase separation have had some successful applications, they are limited by several inherent characteristics which have been typical of such techniques and equipment to date. First of all, the plates and their arrangement has been fixed and static, i.e. once the plates have been put into place, the sole configuration of the separator is determined.
Secondly, it will be understood by those familiar with separation that if any early separation takes place along the laminar flow path, then the characteristics of the mixture being separated will be progressively different as the mixture travels through the separator. Generally speaking, the larger droplets or particles of the discontinuous phase will separate out first, so that the discontinuous phase will progressively contain a larger and larger percentage of smaller and smaller particles; i.e. those which become progressively more difficult to separate. Nevertheless, because the plates are fixed in static position, spacing and configuration, the maximum or optimal configuration for separation is only achieved once and then never again. Additionally, because the particular process forming the mixture may change, the mixture itself may change, thus rendering the separator ineffective.
In short, while the characteristics of a mixture being separated will be constantly changing, the equipment remains the same and is only ideally suited for optimal separation of a mixture with one particular set of characteristics, but will be less than optimal for all other sets of characteristics even as that one discontinuous mixture moves through the separator.
Additionally, because a large number of such plates must be used in close configuration with one another in such laminar flow separators, they are typically made of plastic to reduce their weight and cost and to form a wettable surface with certain discontinuous components, particularly oils. Nevertheless, being an organic material, plastic can be affected by oily mixtures resulting in swelling, distortion, softening and even dissolution of the plates. Additionally, plastic limits the temperature ranges within which the equipment can be operated. Thus, although heating the discontinuous mixture can result in enhanced separation, the limitation of the plastic materials to withstand heat will likewise limit the potential of using temperature adjustment to increase the efficiency of separation. Finally, because the plates are generally molded, static and fixed in position, they can never be adjusted when mixtures of differing characteristics must be separated. In short, any particular typical laminar flow separator is generally suited to separate only one type of discontinuous mixture and even when so suited will only separate that mixture at one particular instantaneous makeup and will be disadvantageous for all other makeups of even the discontinuous mixture that it was designed for.
Most coalescing and laminar flow separation techniques are based, either implicitly or explicitly, upon Stokes' Law which predicts the motion of a particle suspended in a fluid under the influence of the force of gravity. Stokes' Law can be expressed by the following formula: ##EQU1## Where v=the rate at which the particle will settle or rise, called the "terminal velocity"; g=the acceleration of gravity; r=the particle's radius; d.sub.1 =the density of the particle; d.sub.2 =the density of the liquid; and u=the viscosity of the liquid. This basic relationship and other related ones can be expressed in a number of ways which are generally familiar to those aquainted with the scientific bases of fluid behavior.
Where enough turbulence is eliminated to achieve laminar flow, the motion within the continuous phase of a particle or droplet of the discontinuous phase component will be characterized by the resultant of its lateral motion as it is carried along by the laminar flow and its vertical motion described by Stokes' Law. Accordingly, because like materials tend to agglomerate, if the movement of the particles of the discontinuous phase can be encouraged to favor agglomeration, separation will be enhanced. Most laminar flow separators accomplish this with the plates described heretofore.
Nevertheless, like many physical laws (e.g. the ideal gas law), Stokes' Law represents an idealized law which describes the behavior of all such mixtures generally but none specifically. Thus, all real mixtures will differ somewhat from the idealized expression of Stokes' Law, basically because of the difference between actual conditions and the assumptions that must be made in order to even attempt to predict such behavior; e.g. the assumption that particles are spherical, the assumptions which go into figuring viscosity, and a number of other like estimations. Furthermore, Stokes' law only predicts the terminal velocity of a single particle of a single size in the absence of any other like particles. Stokes' law does not deal with real life situations in which many such particles of many sizes are affecting one another, and in which particles move and accelerate at any number of velocities from rest up to their terminal velocity.
To date, laminar flow coalescing-type separation techniques have not attempted to either compensate for, or indeed take advantage of the particular differences between a mixture's actual behavior and its behavior as predicted by Stokes' Law. Consequently, no current laminar flow, coalescence-type separation techniques are ideally suited for more than relatively gross separations, e.g. oil from water, machine scarf from machine fluid, dirt from oil, and the like.
It is thus an object of the present invention to provide a method and apparatus for separating the components of a mixture of a continuous phase and a discontinuous phase in which the continuous phase comprises a liquid which method and apparatus is tailored to the physical, chemical and statistical characteristics of the components of the mixture and which can correlate the configuration of the separation apparatus according to both the initial characteristics of the discontinuous mixture and the intermediate characteristics of the discontinuous mixture as it progressively separates.
It is a further object of this invention to provide a method for analyzing, determining and predicting the characteristics of the discontinuous mixture and its components in order to correlate the configuration of the apparatus to these characteristics.
It is another object of the invention to provide an apparatus which can be progressively correlated to such characteristics as a discontinuous mixture is being separated.
It is a further object of this invention to provide an apparatus which can be used over a wide range of temperatures so that discontinuous mixtures can be heated or cooled as necessary to enhance their separation.